In a typical satellite communication system, satellites may be required to communicate with other satellites to transfer data from a source node to a destination node. A source node or destination node may, for example, be a ground-based cellular telephone. Communication between satellites may be performed by transmitting information from a satellite associated with the source node (hereinafter "source satellite") to a satellite associated with the destination node (hereinafter "destination satellite") or to another intermediate satellite.
A direct communication link between two satellites is referred to herein as a "cross-link". A cross-link is maintained where both the source and the destination or intermediate satellite's communication antennas are pointed toward each other and data communication is occurring. In a satellite communication system where satellites travel in the same direction, only small adjustments to a communication antenna's position are necessary to maintain a cross-link.
During each orbit, a non-geosynchronous satellite will travel in both an ascending (i.e., northbound) and a descending (i.e., southbound) direction. Where multiple, parallel orbital planes exist, satellites in a first orbital plane may travel in the same direction as satellites in an adjacent orbital plane, or the satellites in the first orbital plane may travel in the opposite direction (e.g., satellites in a first orbital plane are ascending and satellites in an adjacent orbital plane are descending). The space between these "counter-rotating" orbital planes is referred to herein as a "seam".
Typical satellite communication systems do not provide a method or apparatus for maintaining a cross-link with satellites in adjacent orbital planes that are traveling in opposite directions. Because of this, a data packet intended for a destination satellite that is traveling in an opposite direction from the source satellite may not be transmitted directly to the destination satellite. The data packet must be transmitted through intermediate satellites traveling in the same or a parallel orbital plane until it reaches an intermediate satellite that is traveling in the same direction as the destination satellite. Then the data packet may be transmitted, potentially through additional intermediate satellites, to the destination satellite. Often times, data packets destined for the other side of a seam must be routed over one of the poles.
Each intermediate satellite that a data packet must be transferred through increases the amount of time it takes for the data packet to travel from its source to its destination. In a communication system that transmits data packets containing voice dam, multiple satellite transfers results in a noticeable delay in receipt of a voice signal. Such delays may be annoying to users of the communication system.
Additionally, routing data packets over the poles may also add traffic congestion on the over-the-pole links that exceeds the link capacity of the over-the-pole links. When communication link capacity is exceeded, data packets must be dropped, thus never reaching their destination.
Therefore, what is needed is a method and apparatus for establishing and maintaining communication links between satellites traveling in different directions so that the number of satellites a data packet must be transferred through to go from a source satellite to a destination satellite are minimized. Minimizing the number of satellite transfers would also minimize the time between origination of a data packet and receipt, thus enhancing system performance. What is also needed is a method and apparatus to increase overall system capacity such that data packets that cannot be transmitted via over-the-pole links may be otherwise transmitted.